Add option textfiltermode=keep-selection

[hemmingsv]

Background:
The textfilter command is great for running a single shell command on a selection (or multiple) from the buffer as stdin. The selection is replaced by the command output, but the output is not selected afterward.

Expected changes:

• Let the user set textfiltermode=keep-selection to keep the output selected. This is great for chaining commands and in line with what I would expect textfilter to do.
• Current behavior is still default (ie textfiltermode=drop-selection)

Personal note: I am new to the code base and never written Go. I find the repo very easy to work with. I might have implemented this in a completely stupid way though. Harsh feedback is welcome. I could have made this into a plugin, but I found it hard to implement the textfilter command in Lua, so went with this native approach instead. If this approach is approved, then I will oversee the documentation as well.

[Andriamanitra]

I think this should be the default behavior. Losing the selection always felt more like a bug than an intentional design choice. Maybe we don't even need the setting?

The implementation seems reasonable to me. I tested it a bit and everything seemed to work alright. There is a bug that causes selections to get messed up when undoing multicursor edits (including the ones done with textfilter), but it already existed before these changes.

I'm on the fence about whether or not it should select the text when you do textfilter without having a selection. On one hand it's nice to see exactly what was inserted but it feels a bit weird for the inserted text to be selected by default.

[hemmingsv]

@Andriamanitra I think that was really good feedback. See new commit where we instead have the options:

• never-select: Old behavior if someone wants to keep that explicitly
• keep-selection: Keep outputs selected it inputs were selected
• always-select: Always select the outputs no matter what (this is the implementation called keep-selection in my previous commit that you commented on).

I also made keep-selection the default, it is the one that makes the most sense as default. Anyone who does not want this behavior can explicitly change to one of the others.

What do you think?

[JoeKar]

Well done.
I am also considering whether we need an option for this, or whether to stay with the actual keep-selection as the default without the configuration parameter.
With the option the user can return to the previous default. 🤔

@dmaluka
Do you have any preferences here?

[dmaluka]

I'm inclined to think we should just make it unconditional. Then if anyone complains about the changed behavior, we can always introduce an option (so that we know there is actually a need for introducing it).

[JoeKar]

I'm inclined to think we should just make it unconditional.

Me too.

@hemmingsv
Would you be so kind and update your PR to just declare the actual keep-selection behavior as default and remove the textfiltermode option? In case we really need to make it configurable we still have the suggestion available in the PRs history. :)

[hemmingsv]

@hemmingsv Would you be so kind and update your PR to just declare the actual keep-selection behavior as default and remove the textfiltermode option? In case we really need to make it configurable we still have the suggestion available in the PRs history. :)

Yes! Less moving parts sounds good. In this case I suppose no documentation changes are needed?

Will push the updated code on Monday.